Radar is a system that uses radio waves to detect, to determine the direction and distance and/or speed of, and to map objects such as aircraft, ships, terrain, or atmospheric precipitation. In a conventional radar system, a transmitter emits radio waves toward a target. The waves reflected by the target back toward the transmitter are detected by a receiver. The receiver typically is in the same location as the transmitter. Although the returned radio signal is usually very weak, radio signals can easily be amplified. Consequently, radar can detect objects at ranges where other emissions, such as sound or visible light, would be too weak to detect. Radar is used in many contexts, including meteorological detection of precipitation, air-traffic control, police detection of speeding ground traffic, as well as various military applications.
The term “RADAR” was coined in 1941 as an acronym for Radio Detection and Ranging. This acronym, of American origin, replaced the previously used British abbreviation “RDF” (Radio Direction Finding). The term has since entered the English language as a generic word, “radar,” that has lost its original capitalization.
One way in which to measure the distance to an object is to transmit a short pulse of radio signal, and to measure the time elapsed for the reflection to return. The distance is one-half the product of round-trip time (because the signal has to travel to the target and then back to the receiver) and the velocity of the signal. This concept was exploited in certain early radars. Since velocity of the signal is the velocity of light, the round-trip time is very short for terrestrial ranging.
Another form of distance-measuring radar is based on frequency modulation. Frequency comparison between two radio signals is considerably more accurate, even with older electronics, than timing the returned signal. By changing the frequency of the returned signal and comparing with the frequency of the transmitted signal, the difference is easily determined. An example is a chirp-radar system, in which the frequency of the transmitted signal is linearly increased during the period of transmission. The chirped signal provides increased signal bandwidth, which is directly proportional to range resolution, i.e., the ability of the system to discriminate between closely spaced targets in the range dimension. Greater bandwidth generally yields higher (better) range resolution.
An “antenna array” in the radar context comprises multiple individual active antennas (antenna “elements”) coupled to a common source or load to produce a directed radiation pattern. Usually, the spatial relationship of the elements in the antenna array contributes to the directivity of the antenna array. The term “active antenna” pertains to an antenna having constituent antenna elements of which the respective energy outputs are modified. Modification of the energy output by an element is usually achieved by coupling the element to an energy source that is other than the mere signal energy passing through the circuit including the antenna element. Another way of achieving modification of the energy output by an element is by coupling the antenna element to a source of energy having an output that is controlled by the signal input. In a phased array of antenna elements, the relative amplitudes of, and constructive and destructive interference effects among, the signals radiated by the antenna elements determine the effective radiation pattern produced by the array. A phased array may be used to point (steer) a fixed radiation pattern, or to scan rapidly in azimuth or elevation.
Antenna arrays are used in several types of radars. One type is called Active Electronically Scanned Array (AESA) radar, also known as an active phased-array radar. In an AESA radar system, the transmitter and receiver functions are performed by numerous small transmit/receive (T/R) modules that are connected to respective antenna elements and that energize the elements simultaneously. Since each element is energized by a respective RF source, the elements are “active.” Another type is called Passive Electronically Scanned Array (PESA) radar, in which a microwave-feed network in the rear of the antenna is powered by a single radio-frequency (RF) source (magnetron, klystron, traveling-wave tube (TWT), or the like). The single RF source sends its waves simultaneously to multiple phase-shift modules that are coupled to respective antenna elements and that energize the elements simultaneously. The phase-shift modules are usually digitally controlled. Since the antenna elements do not receive their RF energy directly from respective modules, the elements are “passive.”
AESA radar systems do not have or utilize a traditional RF source (in the common meaning of the term, including a magnetron, a klystron, a TWT, or the like). These traditional RF sources usually require extremely high operating voltages (reaching 50 kVa). Rather, individual AESA elements create electromagnetic waves using devices such as gallium-arsenide modules that operate at relatively low voltage (e.g., 40-60 volts). In addition, AESA radar systems tend to have simpler mechanical designs and tend to be more compact than other radar systems.
Unfortunately, however, both AESA and PESA antenna arrays require the use of electronic phase shifters, one for each antenna element. This poses the requirement for many electronic units that should remain in a functional state during operation of the radar. Whereas having a large number of independently driven antenna elements provides some accommodation for failures of individual phase shifters and/or antenna elements, the large amount of electronics inevitably increases the probability of failure somewhere.
Synthetic aperture radar (SAR) is a type of radar system in which sophisticated post-processing of radar data is used to produce a very narrow effective beam. SAR can only be used by moving the radar system over a relatively immobile target. Nevertheless, SAR has seen wide applications in remote sensing and mapping. In a typical SAR application, a single radar antenna is attached to the side of an aircraft or the like. A single pulse from the antenna array is rather broad (several degrees) because diffraction requires a large antenna to produce a narrow beam. The pulse also is broad in the vertical direction; often the pulse will illuminate the terrain from directly beneath the aircraft out to the horizon. If the terrain is approximately flat, the respective times at which echoes return allow points at different distances from the flight track to be distinguished from one another. Distinguishing points along the track of the aircraft is difficult using a small antenna array. However, if the amplitude and phase of the signal returning from a given piece of ground are recorded, and if the aircraft emits a series of pulses as it travels, then the results from these pulses can be combined. Effectively, the series of observations can be combined just as if they had all been made simultaneously from a very large, one-dimensional, linear-array antenna; this process creates a synthetic aperture that is much larger than the length of the physical antenna array (and in fact much longer than the aircraft itself).
In SAR, combining a series of radar observations requires significant computational resources. Computations are normally performed at a ground station, after the observations are complete, using Fourier transform techniques. The result is a two-dimensional, reflectivity image of range versus cross range.
A variation of SAR is Inverse Synthetic Aperture Radar (ISAR), which exploits motion of the target (rather than of the radar) to synthesize a large-aperture antenna. ISAR is also used for generating two-dimensional high-resolution images of targets. In situations in which other radars display only a single unidentifiable bright moving pixel of an image, an ISAR image can often discriminate between various missiles, military aircraft, and civilian aircraft. One can generate a high-resolution image of a stationary object by moving an SAR around the object. Alternatively, one can generate the same image using a stationary radar and rotating the object. If the target rotates by a small amount, the rotation has the same effect as if the radar had traveled a distance equal to the arc length at the range R. ISAR can be used for identifying the reflectivity centers of a target with high spatial resolution. A fine two-dimensional reflectivity map of the target is generated using a large-bandwidth transmitted signal to achieve high range resolution and coherently processing the echoes received from different aspect angles of the target. ISAR images of a target region also can be useful for locating scattering regions on the target. ISAR images of a rotating target are produced by processing the resultant doppler histories of the scattering centers on the target. If the target rotates in azimuth at a constant rate through a small angle, scatters will approach or recede from the radar at a rate depending only on the cross-range position (the distance normal to the radar line of sight with the origin at the target axis of rotation). The target rotation will result in the generation of cross-range-dependent doppler frequencies that can be sorted by a one-dimensional Fourier transform. This operation is equivalent to the generation of a large synthetic-aperture antenna formed by the coherent summation of the receiver outputs for varying target/antenna geometries.
3D radar provides radar coverage in three dimensions. Whereas 2D radar provides range and azimuth data, 3D radar provides data concerning range, azimuth, and elevation. Applications include weather forecasting, defense, and surveillance.
While the various conventional radar systems summarized above have numerous advantages, including those noted, their advantageous status is not universal. For example, phased-array radar systems are expensive and complex from both an electronics and signal-processing point of view. With respect to electronics, the need to provide respective phase shifters and control electronics for each element contributes substantially to cost and complexity. With respect to signal processing, during beam steering all the antenna elements in the array are excited simultaneously in the transmit mode. Meanwhile, the phase shifters are continuously energized in a controlled manner to adjust the respective phase of the signal produced by each antenna element. Synthetic aperture radars are limited by, inter alia, having to be mounted on an object (typically an airplane) moving in a controlled manner relative to the target. Also, synthetic aperture radars are not capable of forming three-dimensional images or data.